1. Field of the Invention
The present invention relates generally to the field of x-ray collimators. More specifically, the present invention discloses an x-ray collimator for use primarily in x-ray lithography for semiconductor fabrication.
2. Statement of the Problem
X-ray lithography has been used experimentally in the past for etching semiconductor wafers. However, existing x-ray lithography systems have not been commercially viable due to a number of significant shortcomings, particularly the speed, cost, complexity, and size of such x-ray lithography systems.
For success in a commercial environment, an x-ray lithography system should be able to meet stringent collimation requirements. The x-ray collimator should reduce global divergence to remove all pattern shadowing in the resist, i.e., approximately 3 milli-radians over a 26 mm square mask. Local beam divergence should be reduced to below the level of diffraction in the mask features, i.e., approximately 5 milli-radians. Beam uniformity should be within .+-.5% to 10% across a 26 mm square wafer. The x-ray collimator must also meet gain requirements assuming a 100 watts/ster pinch source to deliver a beam with sufficient flux to support a production rate of 25 to 50 wafers per hour. In addition, the collimator optics must be robust and reliable. The x-ray collimator should not significantly increase the overall cost of the stepper, and should not have excessive space requirements.
The gain requirement is driven by the need for adequately fast systems. If speed were not the driving consideration, then the source could simply be moved back to four or five meters from the mask and meet the divergence criterion. The purpose of the collimator is to provide the low divergence with adequate signal. The speed of the stepper is controlled by the wafer handling time, the step and align time per die, and the exposure time per die. A conventional stepper uses 22 seconds to insert and remove a wafer. It also requires one second per die to step and align. Assuming a typical number of 40 dice per wafer, we can then write an expression for the number of wafers per hour achievable: ##EQU1## where W is the wafers per hour, a is the step and align time in seconds, and e is the exposure time in seconds. Since a is known to be one second, the system would handle 58 wafers per hour with an exposure time of zero. The table below shows the expected system throughput as a function of the exposure time:
______________________________________ Exposure (sec) Wafers/hr Beam (W) ______________________________________ 0 58 .infin. .25 50 .62 1 35 .155 1.5 29 .1 2 25 .077 3 20 .05 4 16 .04 5 14 .031 6 12 .026 7 10 .022 ______________________________________
To achieve 10 wafers per hour requires 7 second exposures, 20 wafers per hour requires 3 second exposures, and 30 wafers per hour requires 1.5 second exposures.
To convert exposure time to beam intensity we must assume a resist sensitivity. As sensitivity is a function of wavelength, we must choose a number that is representative of the speed after convolution with the incident spectrum. For our purposes, an exposure of 23 mJ/cm.sup.2 is reasonable, representing a balance between the 15 mJ/cm.sup.2 at 14 .ANG. and the slower response in the 11 to 8 .ANG. band. It will thus require 155 mJ to expose a 26 mm square. These numbers have been used to generate the third column of the table above.
Inspection of the above table gives some sense of the beam requirements for an x-ray collimator. First, there is no sense in pushing much above one Watt in the collimated beam, because the exposure time has already dropped to a negligible fraction of the time spent on each die. Similarly, a beam with less than about 20 mW will yield below 10 wafers an hour and render the stepper commercially non-viable unless it is quite inexpensive. The present collimator has been designed around these numbers.
A basic tenet of optics is the principle of conservation of brightness. No passive optical system can increase the brightness of a beam, where brightness is defined to be the number of photons per area per solid angle per second. As it turns out, for the pinch source, the brightness of the source is the limiting factor on the beam that can be created. A conventional pinch source generates 100 Watts per steradian of x-rays. These emanate from an area 0.5 mm across, so the source brightness is 400 Watts/mm.sup.2 /s. If the goal is to achieve one watt over a 26 mm square, with no more than 2 milli-radians of divergence, the minimum required brightness is: ##EQU2## Which is the brightness provided by the source. The beam requirement can be relaxed in either power or divergence, or some combination of both. One Watt is achievable, but requires a very efficient collimator. If the collimator has 50% losses, then 0.5 watts is the best possible beam with 2 milli-radians of divergence. If full beam fluxes of one watt or more are to be achieved, then the source must be improved. This can happen either by increasing the x-ray flux (without increasing the size of the emitting spot), or decreasing the emitting spot size without decreasing the emitted flux. For example, in order to achieve 10 Watts and 2 milli-radians, the size of the emitting spot must be held to a stable 0.1 mm. In order to achieve 2 watts and 2 milli-radians, the spot size must be reduced to no more than 0.25 mm. However, the performance level asked of the beam does not reach this until steppers in the 50 to 100 wafer/hour range are required.
The central problem in collimator design is one of angles. Consider that to achieve one Watt in the beam, 1% of the steradian output of the source must be gathered, even if the collimator is 100% efficient. To capture one watt requires an open angle into the beam of 6.degree. width. At the extremes of the entrance cone, the x-rays must be bent through at least 3 degrees. With collimator efficiency included, this grows to at least 5 degrees. To reach upwards to 10 Watts, then involves angles of 15 to 20 degrees. X-rays simply don't bend through such large angles without multiple reflections. These reflections can be in the form of constructive interference from multiple layers, as in crystals and multi-layers, or can be from sequential grazing incidence reflections. This means that the collimator will have either a low efficiency normal incidence reflection, or a series of reflections through co-aligned channels at grazing incidence.
3. Solution to the Problem
None of the prior art show a collimator for use in x-ray lithography that meets all of the conditions for commercially viable in terms of speed, cost, complexity, size, throughput, and collimation tolerances for an x-ray lithography systems. In contrast, the present system is able to meet these stringent requirement using pairs of orthogonal, spherical mirrors in grazing incidence, coupled with flat mirrors at grazing incidence.